Generally in electrophotographic reproduction the original to be copied is rendered in the form of a latent electrostatic image on a photosensitive member. This latent image is made visible by the application of triboelectrically charged toner.
Conventional electrophotographic toner powders are made up of a binder polymer and other ingredients, such as pigment and a charge control agent, that are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder.
The toner thusly forming the image is transferred to a receiver, such as paper or transparent film, and fixed or fused to the receiver. The fusing of toner to receiver can be effected by applying heat, preferably at a temperature of about 90° C. to 200° C.; pressure may be employed in conjunction with the heat.
Application of the requisite heat causes problems such as low printer productivity, receiver damage, excessive energy use, long heat up times for equipment to reach fusing temperature and the like.
The proper fusing temperature is operationally defined as the minimum temperature range above the Tg at which the fused toned image develops sufficient adhesion to the final image receptor to resist removal by abrasion or cracking (see, e.g., L. DeMejo, et al., SPIE Hard Copy and Printing Materials, Media, and Process, 1253, 85 (1990); and T. Satoh, et al., Journal of Imaging Science, 35 (6), 373 (1991)). Minimizing the proper fusing temperature is desirable because the time required to heat the fuser assembly to the proper temperature will be reduced, the power consumed to maintain the fuser assembly at the proper temperature will be reduced, and the thermal demands on the fuser roll materials will be reduced if the minimum fusing temperature can be reduced. The art continually searches for improved dry toner compositions that produce high quality, durable images at low fusing temperatures on the final image receptor.
It has been known for a long time that various additives can be used to modify toner polymer properties and improve fusing. U.S. Pat. Nos. 3,794,594 and 3,980,575, for example, describe the addition of plasticizers. The use of plasticizer in electrophotographic toner compositions beneficially allows formulation of toner particles using materials that otherwise would not be suitable for use in these compositions, because the fusing temperature would otherwise be unacceptably high.
However, there is a problem with the use of plasticizers. When the Tg of the toner is too low, problems such as clumping and flaking in storage, are manifested. Thusly, British Patent GB 2 113 413 describes addition of a plasticizer which has limited solubility in the toner polymer at normal operating, handling and storage temperature. Crystalline aggregates are formed which do not affect Tg. At elevated fusing temperatures the plasticizer melts and induces reduced viscosity of the toner polymer.
U.S. Pat. Nos. 3,488,189 and 3,493,412 describe addition of a solid crystalline plasticizer to the receiving sheet rather than incorporating the plasticizer in the toner particle. Thus, the toners will not be tacky and clump in storage.
Japanese Kokai 2005-208350 describes a polyester resin and a plasticizer which are mixed at 130-210 degrees C. to give toners which are excellent in fixability and endurance.
Russian Patent SU 1769183 describes a toner comprising a styrene-butyl acrylate copolymer, a dye, and a plasticizer which is dioctyl phthalate.